Aircraft icing is known to occur on aircraft wing and tail surfaces when the aircraft is airborne as well as when the aircraft is stationary on the ground. Airborne icing generally occurs on the leading edges of the wings when the surface temperature is at or below freezing. Unfortunately, this can occur at any time of the year when there is moisture or precipitation and when the aircraft is at or above its freezing altitude. Such airborne ice accretion, or buildup, can have deleterious effects on flight performance. Lift decreases, thrust falls off, drag and weight increase, and stall speed dramatically increases. In recent years, undetected airborne icing has contributed to a number of catastrophic crashes and continues to threaten general aviation and high performance commercial jet aircraft.
In the prior art, several ice detection technologies have been developed to measure localized icing. Such technologies include the use of visual observation, vibrating sensing probes, probes with optical sensors, and ultrasonic pulse echo sensors. Visual observation is crude, not suitable for measuring the thickness of ice, and is severely limited by darkness or other poor visibility conditions. Technologies using probes are generally unnecessarily complex and the protruding probes cause undesirable drag. Installation of the pulse echo sensor, on the other hand, requires drilling through the aircraft surface; and such a sensor can only measure localized ice thickness.
More recently, ice thickness measurement techniques have been developed based on measuring the capacitance changes at a surface mounted sensor or electrode produced by the presence of ice buildup or water. See U.S. Pat. No. 5,191,791. With known signal processing techniques, capacitive ice sensors can be used to determine the ice thickness independent of ice composition or temperature. See, for example, U.S. Pat. 4,766,369, which is incorporated herein by reference.
While the recent ice detection technologies have made some advances, even the most current technology can not measure the ice thickness or ice distribution profile of substantially non-uniform ice. Such non-uniform icing has been shown to readily occur on aircraft surfaces and, undetected, can ultimately lead to crashes.